1. Field
The present disclosure relates to a control system and method for simultaneously controlling a fuel concentration supplied to a liquid fuel cell and a temperature of the liquid fuel cell by using a temperature-control based feed-back control without using a concentration sensor, and a fuel cell apparatus using the same.
2. Description of the Related Art
A fuel cell is a power generation system for generating electricity by electrochemically reacting oxygen and fuel, different from existing secondary batteries which store energy.
The fuel cell may be classified into various kinds such as a phosphate-type fuel cell, a solid oxide fuel cell, a polymer electrolyte membrane fuel cell, a molten carbonate fuel cell, an alkali fuel cell or the like, depending on the used electrolyte, and they show differences according to the kinds in their operating temperatures, output scales and applications.
Among them, the polymer electrolyte membrane fuel cell (PEMFC) includes a liquid fuel cell which uses a liquid fuel instead of hydrogen as a fuel.
The liquid fuel cell uses any one of methanol, ethanol, formic acid, isopropanol, propanol, ethylene glycol, dimethyl ether, butanol, water and their mixtures, and electrochemically reacts the mixed fuel with oxygen to directly convert the chemical energy of reactants into electric energy. The liquid fuel cell may be suitable as a future small mobile power source due to high fuel energy density and short charging time.
Chemical Formulas 1 to 3 below show an anode chemical formula, a cathode chemical formula and an entire chemical formula of a direct methanol fuel cell using methanol as a fuel, among the above liquid fuel cells.CH3OH+H2O→CO2+6H++6e−,E0=0.043V  [Chemical Formula 1]3/2O2+6H++6e−→3H2O,E0=1.229V  [Chemical Formula 2]CH3OH+3/2O2→CO2+2H2O,E0=1.186V  [Chemical Formula 3]
As in the above chemical formulas, in case of the direct methanol fuel cell, methanol oxidation reaction and oxygen reduction reaction occurs respectively at both electrodes with the electrolyte being interposed therebetween, hydrogen ions generated as a result of the reaction move from the anode to the cathode through the electrolyte membrane, and electrons move to the cathode through an external electric circuit.
In the liquid fuel cell such as the direct methanol fuel cell, it may give a great influence on the performance and energy efficiency of the fuel cell to supply an optimized fuel concentration to the fuel cell and reuse unreacted fuel discharged from a fuel cell stack.
In other words, when a liquid fuel used in the liquid fuel cell is supplied with a high concentration, a crossover phenomenon may occur so that the liquid fuel moves from the anode to the cathode through the electrolyte membrane, and to this end the fuel crossover may cause various problems such as a performance deterioration of the fuel cell, a decrease in energy efficiency of the fuel, etc.
The crossover phenomenon may be proportional to the fuel concentration. If the fuel concentration is higher than an optimal fuel concentration of the fuel cell, the amount of fuel crossover from the anode to the cathode through the electrolyte membrane may increase, which in turn may lower the performance. To the contrary, if a fuel with a low concentration is supplied to the anode of the fuel cell stack, a fuel deficiency may occur at the anode, which may decrease performance of the fuel cell.
Meanwhile, if a fuel is stored or used in a diluted state inside the liquid fuel cell apparatus, or if a fuel which has been used once and is still containing an unreacted fuel is discharged out and wasted, the energy efficiency of the fuel cell apparatus deteriorates and the advantage as a portable high-density power source may be lost.
Therefore, in order to supply a fuel with a low concentration and increase the energy density of the fuel cell apparatus, the fuel should be recirculated. In addition, in order to maintain the concentration of a recirculated fuel constantly, an undiluted fuel with a high concentration should be supplemented.
For reference, if a reacting solution containing a fuel, namely a diluted fuel, is supplied to the fuel cell stack, the fuel is consumed due to the reaction at the fuel cell stack, and a fuel solution containing an unreacted fuel may be discharged out of the stack. Here, the fuel concentration of the discharged fuel solution lowers below a target concentration. Therefore, in order to maintain the concentration of the fuel in the unreacted fuel solution discharged from the stack in a desired level, namely at a target concentration, an undiluted fuel with a high concentration should be added to the unreacted fuel discharged from the stack to control the concentration. In other words, the undiluted fuel should be supplemented to maintain the concentration of the diluted fuel constantly. For this, in a fuel cell system, a fuel mixer of a certain volume is installed in the fuel circulation system, and an unreacted fuel solution discharged from the fuel cell stack is introduced into the fuel mixer. In addition, an undiluted fuel with a high concentration is injected into the fuel mixer and mixed with the recirculated unreacted solution to make a diluted fuel optimized to have a target concentration, and the concentration-optimized diluted fuel is supplied to the fuel cell stack again.
In this regard, as representative fuel concentration control methods available in the art, there is a control method using a concentration sensor (for example, a methanol concentration sensor). In this control method, the concentration sensor is used to measure a fuel concentration in the fuel cell recirculation system, and while observing the change of concentration, an amount of undiluted fuel required for maintaining the fuel concentration constantly is calculated and supplied to the fuel cell, thereby controlling a concentration of the fuel supplied to the stack.
FIG. 1 is a schematic view showing a general fuel cell apparatus including a concentration sensor.
As shown in FIG. 1, the fuel cell system using a concentration sensor supplies a diluted aqueous fuel from a fuel mixer 1 through a fuel circulating pump 2 to the anode of a fuel cell stack 3.
In addition, regarding the fuel cell stack 3, air is supplied from an air blower 4 to the cathode of the fuel cell stack. The fuel mixer 1 is supplied with an undiluted fuel from an undiluted fuel container 5 through an undiluted fuel supply pump 6.
The undiluted fuel supply pump 6 is connected to a concentration controller 9, and the undiluted fuel supply pump 6 operates according to a signal generated by the concentration controller 9 of the undiluted fuel supply pump. In addition, the fuel mixer 1 is connected to a fuel concentration sensor 17, and the diluted fuel in the fuel mixer 1 is supplied to the concentration sensor 17 by a sensor pump 16 which supplies the fuel to the concentration sensor, thereby measuring a concentration of the diluted fuel.
However, in the conventional concentration control methods, a concentration sensor capable of measuring a concentration of a fuel supplied to the fuel cell should be attached, and separate pipes and a pump for transferring work pieces are required in the fuel circulation system so that the concentration sensor may measure a concentration of the fuel.
For this reason, the conventional concentration control method makes the fuel cell system more complicated and bigger, and the parasitic power losses due to the corresponding sensor and the pump increases, which deteriorate the energy efficiency of the fuel cell system. Further, concentration sensors presently available in the art are expensive and have great measurement errors and short life spans. Therefore, if the concentration sensors are applied to the fuel cell system, the fuel cell system has an increased production cost and lower safety, and therefore may not maintain price and quality competitiveness as a portable power source.
In order to solve the above problems, concentration sensors using electrochemical reactions have been studied and developed.
For example, the concentration sensor using an electrochemical reaction may be fabricated with a lower cost in comparison to an existing concentration sensor. However, as time goes on, the reproducibility and stability of measurement is lowered since the catalyst in the sensor loses its activity.
To solve this problem, a concentration control method without using a concentration sensor is being developed. For example, a method of constantly maintaining a concentration of a diluted fuel injected to a fuel cell by additionally supplying an undiluted fuel as much as the consumed amount of fuel has been developed.
However, according to an observation by the inventors of the present disclosure, this method takes much time to increase a concentration to a target concentration since it does not use a feed-back function and a concentration control method for a startup of a liquid fuel cell in combination. In addition, the rate of change of diluted fuel concentration is slow, and the ability to react to a change of outside environments is not provided.
As an alternative, a method of controlling a concentration of a liquid fuel by injecting a specific amount of liquid fuel during a predetermined period and measuring changes of output power, output voltage or temperature of fuel cell stack, and comparing the changes with already-observed values has been developed.
However, according to an observation by the inventors of the present disclosure, this method has a slow response due to a delay time for periodic observing, does not ensure accurate concentration control, and has an increased error in concentration control if the performance of the fuel cell deteriorates over time due to a long-term operation. In addition, these methods are just applicable to steady-state conditions, where an output current and a temperature of the fuel cell stack reaches to a target value and remains constant.
Meanwhile, methods of controlling a concentration of a fuel by dynamically changing an output current of fuel cell are also being developed.
For example, a method of predicting a concentration of a liquid fuel by intentionally changing the value of the output current density of fuel cell and observing the resultant voltage changing pattern of fuel cell has been proposed.
However, according to an observation by the inventors of the present disclosure, this method also takes relatively long time for controlling a concentration and operating a fuel cell in a normal state, and a temperature and a voltage of the fuel cell stack and a concentration of the fuel continuously vary. In addition, when the performance of the fuel cell deteriorates over time, the concentration control error increases.
Further, while the above methods control a fuel concentration in a normal state operation of a liquid fuel cell without using a sensor, a method of controlling a fuel concentration in a startup of a liquid fuel cell or controlling an output current density is not being developed.
In case of a liquid fuel cell system, in a stratup period of an operation, a fuel concentration is controlled within a great variation range on occasions, and a startup time which is required so that an operating condition of fuel cell reaches a preset target value and a normal state is long.
To solve these drawbacks, a method of determining a fuel feed amount with reference to information of a concentration sensor in a startup stage and an ambient temperature, and controlling the determined fuel feed amount to increase the stack temperature to a target temperature has been developed.
If the ambient temperature is low, the pumping rate of the undiluted fuel is set to be high, and if the ambient temperature is high, the pumping rate of the undiluted fuel is set to be low. The set feed speed of the undiluted fuel is controlled so that the temperature of the stack rises over the set target temperature.
However, according to an observation by the inventors of the present disclosure, this method controls only a fuel concentration in the startup period, and a method for controlling an output current density in the period is not proposed. In addition, since a concentration sensor is used, this method is fundamentally different from a concentration control method not using a sensor, and the variation of ambient temperature which may occur in operation is not considered.
If the ambient temperature increases in operation, a water shortage phenomenon in which water in the water tank is exhausted may occur. If the ambient temperature decreases on the contrary, the heat loss of the stack may increase, which reduces the efficiency of the fuel and the system.